1. Technical Field
The present disclosure relates to image sensors, for example a CMOS sensor such as a high-dynamic sensor, and to a method for reading or controlling the pixels of such a sensor.
2. Description of the Related Art
Conventionally, a CMOS image sensor comprises pixels arranged in rows and in columns. Each pixel essentially comprises a photodiode used in reverse mode, having its junction capacitance discharged by a photocurrent according to a received light intensity, and a circuit for reading the pixel. The measurement of the illumination level received by a pixel is performed by measurement of the voltage across the photodiode at selected times, including the end of a period, called image acquisition or integration period, before and after which the pixel is reset by recharging of its photodiode. The decrease of the voltage across the photodiode during the integration is proportional to the received light intensity. The sensor also comprises a circuit for controlling the read circuit of each pixel, which especially controls the shutting, and a circuit for processing the pixel signal provided by the read circuit of each pixel.
For high light intensities, the photocurrent may be such that the discharge of the junction capacitance during the integration phase adversely affects the measurement. In particular, beyond a given light intensity threshold, which especially depends on the integration time and on the features of the pixel elements, the photodiode reaches, before the end of the integration time, a so-called saturation discharge level, and brightness differences are no longer discriminated.
To improve the sensitivity dynamics of a sensor, that it, the illumination range that a pixel of the sensor is capable of discriminating, it has been provided, in French patent application 2957475 and in patent U.S. Pat. No. 7,586,523, to implement control methods having a double integration period, that is, where the integration period comprises an integration sub-period. The pixel output value is then determined by taking into account the amount of photogenerated charges contained in the pixel at the end of the integration period, and the amount of photogenerated charges stored in the photodiode above a threshold during the integration sub-period.